This invention relates to a composite neutron absorbing material made of copper and enriched boron, and a method of preparing the material with powdered metallurgy techniques.
Neutron absorbing materials are necessary where neutron regulation and management within a neutron producing structure and its surrounding containment are required. Neutron studies at the Argonne National Laboratory are carried out at the Intense Pulsed Neutron Source (IPNS). An accelerator is used to produce a beam of protons for interaction with a uranium target. A spallation reaction is induced in the uranium to produce a neutron spectrum for the physical studies of matter. In order to increase the neutron yield threefold, the depleted uranium target containing small percentages of .sup.235 U has been upgraded by using a target that contains up to 77.5% wt. % .sup.235 U. The neutron yield from the spallation reaction is augmented by fissioning of the target .sup.235 U. This approach requires control of the thermal portion of the neutron spectrum to keep k.sub.eff (fissions per generation .sub.n+1 /fissions per generation .sub.n) &lt;0.85. Thus an effective thermal neutron absorber is required to be located in a limited space in close proximity to the uranium target. The thermal energy developed in the target by proton interaction is removed by water cooling. Safety considerations call for the target assembly to withstand temperatures up to 900.degree. C. if accident conditions are sustained by a loss of coolant.
Previous technology used shielding materials including cadmium and Boral (boron carbides in an aluminum matrix). With the upgraded IPNS, however, a new shielding material was required that could sustain increased neutron loadings, heat approaching 900.degree. C., and yet still be formable to small radii (less than 3 inches). Neither cadmium, boral, nor other materials such as boron carbide (B.sub.4 C) would meet these criteria.
Conventional teaching, such as that in U.S. Pat. No. 3,000,802 to Worn et al., has been that high concentrations of boron cause severe embrittlement of alloys. Thus, a higher boron concentration results in poor ductility, and consequent problems in bending and shaping the metal. Cadmium borate, as suggested in U.S. Pat. No. 2,859,163, is equally unsuitable. It is therefore desirable to develop a material that has an increased capacity for neutron absorption, structural integrity in the face of high temperatures that could melt or damage prior art materials, and ductility that allows for the manufacture of thin, roundly formed shapes.